The availability of high performance optical amplifiers such as the Erbium-Doped Fiber-Amplifier (EDFA) has renewed interest in the use of wavelength division multiplexing (WDM) for optical transmission systems. In a WDM transmission system, two or more optical data carrying channels are combined onto a common path for transmission to a remote receiver. Typically, in a long-haul optical fiber system, the set of wavelength channels would be amplified simultaneously in an optical amplifier based repeater. The Erbium-Doped Fiber-Amplifier is particularly useful for this purpose because of its ability to amplify multiple wavelength channels without crosstalk penalty.
Typically, it is advantageous to operate long-haul transmission systems at high data rates per channel. For example, useful data rates include multiples of the Synchronous Digital Hierarchy (SDH) standard, i.e., 2.5 and 10 Gb/s. As the bit rates increase through the gigabit per second range, the optical powers launched into the transmission fiber need to approach 1 mW per channel. As was demonstrated by Bergano et al. (European Conference on Optical Communications, Brussels, Belgium, paper Th.A.3.1, Sep. 1995) the Non-Return-to-Zero (NRZ) transmission format is particularly useful for transmitting large amounts of data over optically amplified fiber paths. However, NRZ channels operating over long distances require sufficient control over the total amount of chromatic dispersion to ensure low dispersion penalties. Accordingly, the preferred transmission medium for such a system is dispersion shifted optical fibers.
Crosstalk, or the mixing of channels through the slight nonlinearity in the transmission fiber, may arise from the combination of long distance, low dispersion and high channel power. The transmission of many WDM channels over transoceanic distances may be limited by nonlinear interactions between channels, which in turn is affected by the amount of dispersion. This subject was reviewed by Forghieri et al. ("Fiber Nonlinearities and their Impact on Transmission Systems," ch. 8, Optical Fiber Telecommunications, IIIA, Academic Press, 1997). As discussed in Forghieri et al., this problem may be overcome by a technique known as dispersion mapping, in which the generation of mixing products is reduced by offsetting the zero dispersion wavelength of the transmission fiber from the operating wavelengths of the transmitter. This technique employs a series of amplifier sections having dispersion shifted fiber spans with either positive or negative dispersion. The dispersion accumulates over multiple fiber spans of approximately 500 to 1000 km. The fiber spans of either positive or negative sign are followed by a dispersion-compensating fiber having dispersion of the opposite sign. This subsequent section of fiber is sufficient to reduce the average dispersion (averaged over the total length of the transmission system) substantially to zero. That is, a fiber of high negative (positive) dispersion permits compensation by a length of positive (negative) transmission fiber.
While the previously mentioned technique provides effective dispersion compensation, there is a need to better balance the competing factors of reducing the accumulated chromatic dispersion while also reducing nonlinear mixing.